Forced-flow evaporator for compression refrigeration equipment

ABSTRACT

THE INVENTION RELATES TO A FORCED-FLOW EVAPORATOR ASSEMBLY FOR COMPRESSION TYPE REFRIGERATING EQUIPMENT. IN CONNECTION WITH PREVENTING REFRIGERANT IN LIQUID FORM FROM PASSING FROM THE EVAPORATOR TO THE COMPRESSOR, THE EVAPORATOR TUBING IS PROVIDED WITH LIQUID ADSORBING MEANS WHICH COMPRISE A CAPILLARY SYSTEM. THE LIQUID ABSORBING MEANS IS FORMED WITH A GAUZE FABRIC WHICH OCCUPIES LESS THAN ONE-HALF THE CROSS-SECTIONAL AREA OF THE TUBING.

Jan. 19, 1971 z. R, HUE'LLE 3,555,845

FORCED-FLOW EVAPORATOR FOR COMPRESSION REFRIGERATION EQUIPMENT FiledSept. 5, 1968 United States Patent 3,555,845 FORCED-FLOW EVAPORATOR FORCOMPRESSION REFRIGERATION EQUIPMENT Zbigniew Ryszard Huelle, Sonderborg,Denmark, assignor to Danfoss A/S, Nordborg, Denmark, a company ofDenmark Filed Sept. 5, 1968, Ser. No. 757,648 Claims priority,application Ggrmany, Sept. 6, 1967,

Int. cl. F2 51 41 /04 US. Cl. 62-225 6 Claims ABSTRACT OF THE DISCLOSUREThis invention relates to a forced-flow evaporator for compressionrefrigeration equipment incorporating a temperature sensor which isarranged in the vicinity of the evaporator output point and whichcontrols an element that influences the flow of refrigerant, for examplea thermostatic expansion valve fitted ahead of the evaporator.

In refrigerating equipment of this kind liquid refrigerant must beprevented from passing from the evaporator into the compressor or itscase, since otherwise trouble could occur. As soon as the temperaturesensor determines that the evaporation temperature, rather than thesuperheating temperature, obtains at the point of measurement, i.e. thatliquid refrigerant has still to be evaporated, the flow of refrigerantmust be throttled or interrupted. For reasons of safety, the elementthat influences the flow of refrigerant must, how ever, respond if aprescribed superheating temperature is not reached, and should notrespond only when the evaporating temperature is attained. Practice hasshown that the superheating temperature at which response occurs must behigher than the evaporating temperature, even if, for precautionaryreasons, a re-evaporation run is provided to enable only refrigerantvapour to return to the compressor. This fact, however, runs counter tothe requirement that evaporation should be caused, as far as possible,to take place over the entire length of the evaporator, since theevaporator and the refrigerating system are best utilized in this way.Optimally, such control of the interposed expansion valve, by thetemperature sensor, should be such that just enough liquid refrigerantis introduced into the evaporator for it to be completely evaporatedexactly at the end of the evaporator.

In the case of absorption refrigerating machines, it is known to use aheat-exchanger wherein a corrugated and helically wound wire gauze isinserted in a tube in order to increase the turbulence of therefrigerant vapour flowing through and thereby to increase theheat-transfer by conducting heat along the wire gauze to the wall of thetube. Also, the gauze is able to retain liquid refrigerant by capillaryaction until it is evaporated. Such an ar rangement is not suitable forcompression refrigerating equipment since the wire gauze, which occupiesthe entire cross-section of the tube, would cause a drop in pressure,greatly reducing the efficiency of the refrigerating equip- 3,555,845Patented Jan. 19, 1971 ment, at the high flow velocities, which can beas high as 5 metres/sec. in the case of the refrigerant vapour.

Also known is an evaporator for absorption refrigerating equipment,wherein a tubular surface is provided with a longitudinal groove andwith closely spaced circumferential channels of such size that liquidcan be drawn into them by capillary action. In this way, the surface ofthe evaporator tube is intended to be uniformly wetted even if theliquid refrigerant, subjected to no pressure, drips on to it only at oneor a few places.

The object of the present invention is to provide a forced-flowevaporator for compression refrigerating equipment, the control systemof which responds to a small excess-heat temperature and wherein it isnevertheless ensured that no fluid refrigerant reaches the compressor.

According to the invention, this object is achieved by providing atleast part of the wall of the evaporator passage with means forincreasing wetting by liquid refrigerant, which means, however, leavethe major part of the cross-section of the passage open.

During evaporation in a forced-flow evaporator, vapour is formed whichflows through the piping together with the liquid. The liquid proportionprogressively decreases until an evaporation end-point is reached beyondwhich only vapour is present. It is known that even in the case ofconstant flow, the evaporation end-point moves to and fro, in periods ofup to 3 sec., over a considerable length of the evaporator piping, e.g.,50 cm. The invention is based upon the knowledge that the excess-heattemperature, hitherto required for reasons of safety, can beconsiderably reduced if the migration of the evaporation end-point alongthe evaporator piping can be curtailed.

Such curtailment of the migration is achieved by the means forincreasing the wetting. These lead to larger quantities of liquidadhering to the wall of the passage, i.e. to a larger surface of thepassage wall being wetted. This results in improved heat-transfer andmore rapid evaporation. Furthermore, consequent upon the measurementsurface tension, a large part of the liquid adheres to the wall of thepassage and is not so readily displaced by the pressure-difference thatexists. This applies particularly in the case of the ring of liquid thatforms on the wall of the passage on completion of evaporation. For allthis, the cross-section of the passage is substantially open, so thatthe remaining flow, particularly that of the vapour, is not adverselyafiected.

In view of these effects, the means for increasing the wetting should belocated at least in the zone near the point at which evaporation ends,i.e., in the portion in front of the sensor. They need only extend overroughly the second half of the length of the evaporator, for example. Inmany cases, particularly where the system is to be fitted at a laterstage, it is of advantage if the main part of the evaporator is providedwith an added pipe, to the end of which the sensor is fitted, and themeans for increasing the wetting extend only along this pipe.

In a preferred form of construction, the means for reinforcing thewetting action take the form of a capillary system. The liquid israpidly spread over a large surface of the wall of the passage by thecapillary system and is efiiciently held by surface tension.

A particularly simple form of the invention is characterized by aninserted wall, containing orifices and extending parallel with the wallof the passage at a short distance therefrom and supported thereon, sothat a space is created between the inserted wall and the passage wall,this forming the capillary system. In this way, a relatively thick layerof liquid is spread over and retained on the wall of the passage.

The inserted wall and its supporting means are preferably formed from agauze fabric which lies on the passage wall, e.g. the wall of the tube.Such a gauze material can be readily inserted into a tube. Since theliquid can flow on both sides of the gauze, particularly good wetting ofthe wall of the passage results. The cross-section of the passage isnot, however, interfered with to any considerable extent at the centre.The wire gauze can be readily provided with a capillary mesh so thatliquid is retained therein in a particularly efficient manner.

It is of advantage to provide an axial gap in the gauze fabric. Bycompressing the gap, the gauze fabric can be readily introduced into theflow-passage of the evaporators. In the case of evaporators in which oilis likely to be deposited, the gap should be located at the bottom ofthe cross-section of the passage, so that the oil can drain off.

The invention will now be explained in more detail by reference to thedrawing, wherein:

FIG. 1 shows, schematically, a first embodiment of the invention,

FIG. 2 is a cross-section through the evaporator tube in the region ofthe line A-A, and

FIG. 3 is a schematic illustration of another embodiment.

FIG. 1 shows a normal tubular evaporator, which consists of a tube 1, atthe front of which is fitted a thermostatic expansion valve 2 and at theoutlet of which is fitted a temperature sensor 3. By means of thissensor and with the help of the expansion valve 2 the flow ofrefrigerant is so adjusted that, whatever the circumstances, all theliquid refrigerant is evaporated at the point 4. In known evaporators,the point at which evaporation ends ranges over quite a considerablelength of the evaporator, for example between the points 4 and 5. At themoment at which evaporation used to be completed at point 5, therefrigerant vapour is considerably over-heated along the stretch runningto the sensor 3. Below this excess-heat temperature, the thermostaticsystem 2 and 3 should not respond, since when the supply of refrigerantis correspondingly increased, the end point 4 moves beyond the sensor 3and liquid refrigerant would thus reach the compressor.

According to the invention, a cylindrical gauze element 6 is inserted inthe smooth-walled evaporator tube 1. In the embodiment illustrated, thisgauze extends over approximately the second half of the length of theevaporator, i.e. over about 40-60% of the length. By this measure, thepoint at which evaporation ends is restricted to moving backwards andforwards over a very much smaller distance, for example, between thepoints 4 and 7. The possible overheating between point 7 and the feeler3 is considerably smaller than hitherto. Consequently, the thermostaticsystem may respond at a lower excess-heat temperature, without therebeing any danger of the liquid refrigerant reaching the compressor.

FIG. 2 shows the cylindrical gauze element 6 inserted in the evaporatortube 1, the wires 8 of this element also forming supports 9 (indicatedschematically). The gauze contains a longitudinal slot 10 which, on theone hand, can serve for the discharge of deposited oil and, on the otherhand, enables the gauze to be easily pushed into the tube 1 in front ofthe bend of this tube. The gauze lies around the smooth inner wall ofthe tube 1 and forms a space 11 which is of such size that refrigerantcan be retained therein by capillary action or surface tension. Therefrigerant is, of course, driven along the tube by the compressionpressure. On account of the uneven surface of the wire gauze and thecapillary adhesive force, the layer of liquid refrigerant flowing inthis area wets the wall of the passage very efliciently, and thiscontributes to a reduction in the fluctuation of the point at whichevaporation terminates. Due to this wetting, the mobility of the ring ofliquid is reduced. Furthermore, the heat-transfer is improved andevaporation is thus accelerated. This acceleration of the evaporationalso reduces the fluctuations in the point at which evaporation ends,since the small quantity of liquid finally remaining is evaporated overa shorter portion of the tube.

In the embodiment shown in FIG. 3, there is shown a normal plateevaporator 12 in which the evaporator pipe 13 has been made by theroll-bond process. In order to be able to apply the concept of theinvention while still utilizing this simple manufacturing process, atube 14 is added to the plate evaporator 12, a gauze 15, equivalent tothe gauze 6 shown in FIG. 2, being inserted in this tube. The sensor isthen fitted at the end of the tube 14.

The mesh-size of the gauze will depend upon the particularcircumstances. Mesh-size number (DIN 4189), for example, has proved tobe particularly suitable. A capillary system can also be obtained bymachining the inside wall of the pipe; the insertion of a sock-likegauze element is of advantage, however, both for reasons of simplermanufacture and for reasons of its superior functioning.

What is claimed is:

1. A forced-flow evaporator assembly for compression type refrigeratingequipment, comprising, tubing means having inlet and outlet ends, valvemeans at said inlet end and temperature sensing means at said outlet endfor controlling said valve means, liquid absorption means in said tubingmeans comprising a capillary system for absorbing liquid refrigerant,said liquid absorption means occupying less than one-half the linearlength of said tubing means, said tubing means defining an internal wallsurface, said liquid absorption means comprising a perforated wallinternally spaced from said wall surface to define an annular spacebetween said wall surface and said wall, said annular space forming aportion of said capillary system.

2. A forced-flow evaporator assembly according to claim 1 wherein saidperforated wall comprises a gauze fabric spaced for said wall surface.

3. A forced-flow evaporator assembly according to claim 2 wherein saidgauze fabric is a heat conducting material.

4. A forced-flow evaporator assembly according to claim 1 wherein saidperforated wall defines a longitudinally extending channel.

5. A forced-flow evaporator assembly according to claim 1 wherein saidliquid absorption means is disposed between said temperature sensingmeans and said valve means and in juxtaposition to said temperaturesensing means.

6. A forced-flow evaporator assembly according to claim 5 wherein aboutone-half the length of said tubing,

means is equipped with said liquid absorption means.

References Cited UNITED STATES PATENTS 2,446,763 8/1948 Haymond 62-5272,702,460 2/1955 Gaugler 62527X 2,720,763 10/1955 Doekeli 625272,983,107 5/1961 Forrest 6'2527X MEYER PERLIN, Primary Examiner US. Cl.X.R. 62-527

